1. Field of the Invention
This invention relates to a soft focus lens system, and more particularly to a soft focus lens system for use in photography wherein the image quality is capable of being altered to achieve a desired degree of softness as a result of varied spherical aberration of the lens system.
2. Brief Description of the Prior Art
Soft focus lens systems have been used in photography for a considerable period of time. Various different soft focus lens systems have been suggested such as those disclosed in U.S. Pat. No. 2,233,591; U.S. Pat. No. 1,370,885; U.S. Pat. No. 3,843,235; U.S. Pat. No. 3,397,023; U.S. Pat. No. 3,045,530; U.S. Pat. No. 2,959,105; U.S. Pat. No. 3,476,457; U.S. Pat. No. 1,463,132 and U.S. Pat. No. 1,446,634. U.S. Pat. No. 2,381,098 discloses a lens system wherein movement of the lenses may introduce or remove softness of an image.
One of the known ways of designing a soft focus lens system is to intentionally increase the amount of spherical aberration beyond the normal tolerance limits acceptable in sharp focus photography.
Some soft focus lenses known in the prior art have been designed to permit variation of the spherical aberration of the lens system thereby allowing the photographer to obtain a desired degree of softness of a photographic image.
Such prior art lenses with variable spherical aberration usually comprise a three-group lens system having an intermediate negative lens group, shiftable along an optical axis, and a lens system including a shiftable front element, the latter being disclosed in British Pat. No. 198,569, published June 7, 1923.
Generally in order to obtain a soft focus image, it is sufficient to generate only a large spherical aberration, with other undesirable optical phenomena such as astigmatism, coma and other aberrations preferably being corrected as it is conventionally done in ordinary photographic lenses. The prior art systems have not enabled one to obtain an entirely satisfactory soft-focus picture because varying the spherical aberration is usually accompanied by deteriorations in other aberrations, thus rendering a resulting picture more or less indistinguishable from a picture taken with a poorly compensated lens. Furthermore, since shifting of any lens group to vary the spherical aberration brings the lens system out of focus, it has been necessary to refocus the lens system every time a change in the spherical aberration has been made as the lens system was adjusted to obtain the desired degree of softness. It can be readily appreciated that adjustment of the focus of the entire lens system can be difficult when viewing a soft tone image through a viewfinder.
The ordinary soft focus lens of the prior art having a preset non-variable spherical aberration, is always in a soft-focus condition and, as a result does not lend itself well to focusing when it is used as a lens for a single reflex camera.
Improvements in the design of soft focus lens systems including the feature of variable spherical aberration were suggested by U.S. Pat. No. 4,124,276 issued on Nov. 7, 1978 which is assigned to the same assignee as the present application. The variable soft focus lens systems disclosed in that patent require however, that at least two lens groups be shifted at different rates along the optical axis as a variation in the spherical aberration is made, as shifting only one of the lens groups so as to create a soft image would result in an unacceptable defocusing of the image. Shifting two or more lens groups at a differential speed, as it is necessary in that system, however requires an elaborate mechanism.